A wide variety of devices for cutting and removing bone tissue are known in the air. Examples of such include those described in U.S. Pat. No. 5,269,785 issued to Bonutti, U.S. Pat. No. 4,830,000 to Shutt, and U.S. Pat. No. 5,190,548 to Davis. In general, these and similar devices utilize a rotating cutting tip similar to a drill displaced at the distal end of drive shaft. Bone cutting devices for use in reaming the medullary canal typically use a flexible drive shaft because the medullary canals of bones are seldom straight and usually will have some degree of curvature. Most reamers also have a central bore through both the reamer and the drive shaft. The central bore is intended to receive a long, small diameter guide pin or wire which is initially inserted into the medullary canal to act as a track for the advancing reamer.
Reamers are used in orthopedic surgery to prepare the medullary canals of bone for a wide variety of surgical procedures. Such procedures include total hip and knee replacement, nail insertion to stabilize a long bone fracture, an intramedullary osteotomy, and bone harvesting for grafting purposes.
From both a mechanical and a biological point of view, medullary reaming is particularly beneficial in improving the performance of implants. Specifically, reaming expands the medullary canal so that larger diameter implants can be inserted. These larger diameter implants are less likely to fail. In fact, certain fractures require over-reaming so that larger implants can be used. Without reaming, the surgeon must use a "best guess" estimate when selecting the diameter of the implant. The medical literature contains numerous case studies reporting the adverse consequences of an inaccurate estimate. Reaming provides a direct measurement of the diameter of the medullary canal, and thereby allows for the selection of an implant that precisely fills the canal. As a result, the stability of the fracture site is enhanced by achieving endosteal contact. When implants do not fill the medullary canal, load sharing between the implant and the bone is decreased. This increases the load that is transferred to the implant and promotes both implant failure and stress shielding of the bone.
Despite such benefits, negative consequences have also been associated with medullary reaming. In particular, current procedures for reaming the medullary cavity can result in an increase in both temperature and pressure. Like any process in which material is being removed, reaming causes generation of heat. Furthermore, a hydraulic pressure, which far exceeds that of blood pressure, builds up in the cavity during reaming. The reamer acts as a hydraulic piston within the bone cavity, and if the contents of the canal, which include a mixture of medullary fat, blood, blood clots, and bone debris, enter the blood stream, an embolism can result. Excessive heat has been associated with an increased incidence of aseptic necrosis of the cortex and elevated pressure has been associated with an increased risk of fat emboli. These complications are more likely to occur in patients when extenuating factors such as shock, existing lung contusion, multiple traumas, or pre-existing pulmonary impairment are present. In these situations, the preferred method of reaming would usually not be performed due to the increased risks involved.
Various devices and methods exist for reducing the intramedullary pressure build-up during reaming. For example, in prosthetic joint replacement, a distal venting hole, a large insertion hole, and a modified technique for cement insertion have all been shown to have some success ill reducing pressure, and presumably, the chance of fat embolism. Venting holes in the bone only have little effect because their diameter is typically too small and local peak values must be assumed during the passage of the reamer. Similarly, reaming the medullary cavity less does not prevent pressure increase. In fact, pressure can be high even for reamers of small diameter.
Another technique which has been used in an attempt to reduce temperature and pressure is to perform the reaming in multiple steps with increasing size of reamers with each step. As a result, reaming procedures are done slowly with the application of gentle pressure and requiring multiple passes. Usually reaming is performed in 1 mm diameter increments until the bone cortex is reached and then in 0.5 mm increments thereafter. In this regard, the reaming is carried out with less compression force and the intramedullary pressure can be easily reduced with most reaming devices utilizing this slow process. A faster reaming process utilizing fewer passes would be desirable in order to reduce operating time and medical costs.
Another disadvantage associated with current devices and methods is the reuse of reamers. Because current methods involve the use of multiple reamers of variable sizes to create one large opening in the medullary canal, reamers are usually reused in subsequent bone reaming procedures. As a result, reamers may become blunt over time and their continued use can produce greater intramedullary pressures and a greater increase in cortical temperature. Consequently, the careful attention of surgeons and operating staff to treat the reamers gently and replace them whenever necessary is trying and costly. A single use device is desirable to avoid the problems associated with the dulling of reamers which occurs with time.
Another disadvantage of current devices is due to the use of reamer designs with shallow flutes and large shafts. It has been shown that reamers with small shafts and deep flutes are more beneficial in reducing intramedullary pressure and temperature.
Thus, there exists a need for a device and method for reaming a medullary canal at an enhanced rate without increasing the risk of fat emboli and heat necrosis upon cutting and removal of bone tissue.